Offshore underwater storage tank

ABSTRACT

An offshore storage tank, to be placed on a sea floor, having a dome-shaped rigid continuous enclosure, open at the bottom, a buoyant chamber in the enclosure extending over a large part of the area of the enclosure and positioned and sized to provide buoyancy balance or equilibrium to the tank when the tank is floating on water with or without any amount of air in the large volume defined by the enclosure, a conduit to remove air from beneath the enclosure to lower it in water. The enclosure can constitute two spaced apart rigid domed shells with rigid connecting members joined to each shell to hold them in fixed position relative to each other, with a concrete ring closing the bottom of the enclosure between the shell ends thus defining a hollow enclosed space between the shells.

United States Patent 9] Stafford et al. A v

[ July 23, 1974 1 1 OFFSHORE UNDERWATER STORAGE TANK [75] lnventors:Donald Claude Stafford, Hinsdale; Gerald Edward Burns, Villa Park; KerryCharles McKenna, Oak

Brook, all of 111.

[73] Assignee: Chicago Bridge & Iron Company, Oak Brook, 111.

[22] Filed: Jan. 17, 1972 [21] Appl. No; 218,357

[52] U.S. Cl. 114/05 T, 61/46 [51] Int. Cl B63b 35/44 [58] Field ofSearch 114/05 R, 0.5 T, 16 E, 114/65 A; 61/46, 46.5

[56] References Cited UNITED STATES PATENTS 1,859,322 5/1932 Wilson114/0.5 T 3,385,464 5/1968 Courbon 6l/46.5 3,429,128 2/1969 Stafford etal. 1l4/0.5 T X 3,686,875 8/1972 Morgan 61/46 3/1973 Arne et a1 6l/46.5

Primary ExaminerTrygve M. Blix Assistant Examiner-Barry L. KelmachterAttorney, Agent, or Firm-Merriam, Marshall, Shapiro & Klose [5 7]ABSTRACT An offshore storage tank, to be placed on a sea floor, having adome-shaped rigid continuous enclosure, open at the bottom, a buoyantchamber in the enclosure extending over a large part of the area of theenclosure and positioned and sized to provide buoyancy balance orequilibrium to the tank when the tank is floating on water with orwithout any amount of air in the large volume defined by the enclosure,a conduit to remove air from beneath the enclosure to lower it in water.

The enclosure can constitute two spaced apart rigid domed shells withrigid connecting members joined to each shell to hold them in fixedposition relative to each other, with a concrete ring closing the bottomof the enclosure between the shell ends thus defining a hollow enclosedspace between the shells.

1 Claim, 9 Drawing Figures PATENTEDJUL 2 319% sum 1 nr 3 1 OFFSHOREUNDERWATER STORAGE TANK This invention relates to tanksfor storingliquids. More particularly, this invention is concerned with an offshoreliquid storage tank which can be floated to a site and submerged.Although other liquids may be stored which have specific gravities lessthan the specific gravity of water, the tank is particularly useful forstoring oil and liquid hydrocarbon products derived therefrom.

Large amounts of oil are obtained from offshore wells many miles fromthe nearest shoreline. Because of the water depths encountered inoffshore oil production, as well as surface and underwater currents andstorm potentials, the provision of storage tanks at the site haspresented many engineering problems. The large size of the tanks, andthe problems involved in building them offshore, has so far made itessential that they be built completely, or in major components, on landand transported to the offshore site for positioning or erection.

Transporting tanks to an offshore site has usually required floatingthem, withall the problems of buoyancy and stability, plus structuralintegrity, that this involves. Upon arriving at the site, the tanks mustbe lowered until they are supported by, or rest upon, the floor or bedof the body of water. As tank buoyancy is decreased, floating stabilityvaries, with capsizing a constant threat. Thus, experience has shown adefinite need for offshore oil storage tanks which can be floated to asite with stable buoyancy and then submerged to be supportedby the floorof the body of water while maintaining control of the tank againstunstable dangerous listing or tilting and with full control of thedescent.

There is accordingly provided by this invention an offshoreliquidstorage tank, floatable to a site for positioning on the floor ofa body of water, of. such design and shape as to possess unusuallystable buoyancy while floating and when submerged. The tank can be builtof a size and capacity considered most suitable for a particularoffshore location. Strength against underwater turbulence and pressureis also a feature of the tank provided by this invention.

The offshore storage tank of this invention has a generally dome-shapedrigid continuous enclosure, open at the bottom, surrounding or defininga large volume. The dome-shaped enclosure can be of any suitable shapewhich will define a volume therebelow. In horizontal section, it can becircular or polygonal or a com- I bination of straight and arcedsegments. In vertical section through its axis, the dome-shapedenclosure can be conical, a circular or elliptical segment or acombination thereof. However, the peripheral edge of the dome-shapedenclosure advisably lies in a horizontal plane and is adapted to rest ona' sea floor.

The dome-shaped enclosure has integral buoyancy means extending over alarge part of the area of the enclosure. The buoyancy means ispositioned and sized to provide buoyancy balance or equilibrium to thetank when the tank is floating on water, with or without any amount oftrapped air beneath the enclosure. Means is also included to remove airfrom beneath the enclosure to lower the tank into the water. Inaddition, means is provided to add ballast to, and remove air from, thebuoyancy means to submerge the tank for lowering it to the floor of abody of water.

FIG. 1 is a vertical sectional view of an offshore tankhaving anenclosure comprised of two concentrically arranged semispherical shells;

FIG. 2 is an offshore storage tank having an enclosure which issemispherical and in which the buoyancy means comprises spaced aparthorizontally positioned hollow tubular ribs, and vertically positionedhollow tubular ribs which radiate outwardly and downwardly from the topof the enclosure;

FIG. 3 is an enlarged view, partially in section and partially brokenaway, illustrating in greater detail the enclosure structure of FIG. 2;

FIG. 4 is a sectional view taken along the line 4-4 of FIG. 3;

FIG. 5 is an elevational view, partially broken away, of an offshorestorage tank in which the enclosure has tubular buoyancy ribs whichradiate outwardly and downwardly from the top central portion of theenclosure;

FIG. 6 is an offshore storage tank in which the enclosure is a dome-likeshape having a plurality of spaced apart hollow tubular ribs or toroidalelements for buoy- FIG. 7 is an elevational view, partially in section,of an offshore storage tank in which the enclosure comprises two spacedapart semispherical shells with a tubular column mounted thereon andprojecting to above sea level;

FIG. 8 is an elevational view, partially in section, of an offshorestorage tank in which the enclosure comprises two spaced. apartconcentric semispherical shells having bulkheads between the shells todivide the space therein into separate horizontally positioned airtightcircular compartments; and

FIG. 9 is an enlarged vertical sectional view showing the bottom portionof the closure of. FIG. 8.

Each of the enclosures shown in the drawings is essentially circular inhorizontal section and is open at the bottom. The enclosures are rigidand made of metal unless otherwise stated.

With reference to FIG. 1, the offshore storage tank 10 has an enclosure11 which has an outer semispherical metal shell 12 and an innersemispherical metal shell 13 concentrically positioned with respect toouter shell 12. A plurality of internal braces 14 maintain the outer andinner shells 12 and 13 in fixed position relative to one another. Theshells l2 and 13, as well as the internal braces 14, are madesufficiently strong to withstand the pressure exerted thereon when thetank ispositioned on the sea floor at a locality for which the tank isdesigned. Enclosure 1 1 can be pressurized with air to aid shells 12 and13, as well as internal bracing 14, in withstanding loads. Although itis not essential, it is highly advisable to provide a substantialballast at the lower bottom edge of enclosure 11. Thus, as shown in FIG.1, concrete 15 is placed between outer shell 12 and inner shell 13 inthe lower bottom portion between such shells. The result is a ballastring which extends completely around the circular bottom edge ofenclosure 11. Furthermore, the lower edge is advisably pointed or madewedge-shaped to facilitate penetration into the sea floor.

Conduit 16 is provided in the top portion of enclosure 11 andcommunicates with space 17 which is enveloped by the enclosure. Valve 18is provided in conduit 16 to control flow through the conduit. Conduit19 communicates with the bottom portion of the space or volume 17defined by enclosure 11 and valve 20 controls flow through conduit 19.

A tank such as described with respect to FIG. 1 can be built onshorenext to the edge of a sea in a suitable graving dock. After the tank hasbeen built, the graving dock can be flooded and the tank floated to sea.The tank will not have a substantial draft because it will floatprimarily on the air bubble which occupies the space or volume 17enveloped by enclosure 11. Furthermore, the tank will have excellentstability, even when it is constructed with the center of gravity abovethe center of buoyancy because of the wide cut-water plane of enclosure11. Because of the width of the enclosure wall, whether at the locationof the concrete ballast 15 ring at the bottom edge, or the thickness ofthe wall at the spaced apart shells 12 and 13, an effective rightingmoment is provided when the tank floats on the air bubble envelopedbeneath the enclosure. If the enclosure was a single shell of smallthickness, the cut-water plane would be so small as to provide verylittle righting moment and the tank would go into a state of instabilityand tilt out of control and capsize.

To submerge the tank shown in FIG. 1, air is removed from the space 17beneath enclosure 11 by means of conduit 16 with valve 18 open. Valvecan be kept closed because water flows into the lower part of space 17from the surrounding sea when air is vented through conduit 16. Thecontinued removal of airthrough conduit 16 results in a progressivelowering or submerging of the tank into the sea. Such lowering howeveris achieved with total stability of the tank. When all of the air inspace 17 is removed, valve 18 can be closed although if left opensubmerging of the, tank is not adversely affected.

A tank of the general shape and construction as shown in FIG. 1 willusually have the center of buoyancy above the center of gravity-when thetank floats with minimum draft on the sea surface. As air isremoved fromspace 17, the center of buoyancy moves with respect to the center ofgravity until the center of gravity is below the center of buoyancy. Assubmergenceof the tank continues with removal of air from space 17', thecenter of buoyancy continues to migrate higher above the center ofgravity with the result that v the tank remains stable at all timesduring removal of air from space 17." V

Enclosure 11 is constructed so that when all air is removed from space17, the enclosure will be near neutral buoyancy but will still possess asmall residual buoyancy. Such residual buoyancy is provided by the hardvolume space between outer shell 12 and inner shell 13. Such hard volumemust be sufflciently large to provide the residual buoyancy required toovercome the effective-weight of the tank components when the tank isessentially all below the surface of the sea. At such point the tank canbe likened to an iceberg in that only a small part of the tank remainsabove the level of the sea. To complete submergence of the tank fromsuch position, cables can be used to pull it to the bottom. Thus, cablescan be run from the tank to anchored pulleys on the sea floor and fromthere to a winch on a barge on the sea surface or onto a tower placed onthe tank itself. Once the tank has been cranked down onto the sea floor,it can be secured in position by means of piles or simply by filling thespace between the outer shell 12 and inner shell 13 with a suitableliquid or solid ballasting material. Thus,.the space between the twoshells can be flooded with sea water or alternatively such space can bepartially or completely filled with drilling mud, concrete, gravel, sandor some other suitable heavy solid material.

After the tank of FIG. 1 is placed on the sea floor, it can be used forthe storage of oil. Valve 20 can be left open for sea water to flow inand out of space 17. When oil is fed to space 17, it floats on the waterand forces sea water out through conduit 19. When oil is removed fromthe tank, sea water flows in through conduit 19. Thus, the entire space17 is always full of liquid.

FIG. 2 illustrates another embodiment of enclosure which can be employedfor an offshore liquid storage tank. Enclosure 25 has a plurality ofspaced apart hollow tubular ribs or members 26 which radiate downwardlyfrom a central top area of the enclosure. Tubular members 26 are hollowand are made sufficiently strong to withstand the pressure of the seawhen the enclosure rests on the sea floor. The generally domeshapedenclosure 25 also has a plurality of horizontally positioned spacedapart hollow tubular ribs or members 27 which intersect with, andcommunicate internally, with radially located tubular members 26. FIGS.3 and 4 show in somewhat greater detail the intersection of the tubularmembers 26 and 27. The area between spaced apart adjacent tubularmembers 27 and between spaced apart adjacent tubular members 26, isfilled by plate members 28 welded to the tubularmembers 26 and 27.Conduit 29 communicates with the upper interior space 30 encompassed orenveloped by enclosure 25 so that air may be vented from such spacethrough valve 31 during submergence thereof. Conduit 32 communicateswith the lower interior space or volume-enveloped by enclosure 25 andprovides a means for flow of sea water into and out of such space. Valve33 is provided to control such sea water flow through conduit 32.

. Enclosure 25 floats with inherent stability on water because of thesubstantial cut-water plane created by the tubular members 26 and 27when they are immersed in water. It should be understood that the sizeof the tubular members 26 and 27 must be adequate to provide a cut-waterplane righting moment sufficiently large to bar-uncontrolled tilting orcapsizing of enclosure 25. If desired, the lower part of tubular members26 and one or more of the lower horizontal tubular members 27 can befilled with ballasting material to lower the center of' gravity of theenclosure to further increase its inherent stability.

The enclosure of FIG. 2 can be submerged in the same manner as describedwith reference to the structure of FIG. 1. Appropriate conduits andvalve means can be provided to selectively add ballasting material,either solid particulate material or a liquid, to any or all of tubularmembers 26 and 27. Appropriate partitions or bulkheads can also beinserted at any suitable location within tubular members 26 and 27 toretain such ballasting material in predetermined locations in order thatthe ballast will be maintained in position and not slide or flow to oneside of the enclosure and create an imbalance with loss of stabilityduring floating and submerging.

FIG. 5 illustrates another embodiment of enclosure which can be used foran offshore liquid storage tank. Enclosure 40 is generally dome-shapedand has a plu- 5. rality of metal tubular ribs or members 41 whichradiate outwardly and downwardly from a central top portion of theenclosure. The central top portion of enclosure 40 has a flat metalplate 42, circular in shape, which is joined to a toroidal or circulartubular member 43 from which tubular ribs 41 project outwardly anddownwardly. Metal plates 44 join adjacent radial tubular members 41 inrigid fixed spaced apart position. Conduit 45 communicates with theinterior upper space enveloped by enclosure 40 so that air can beremoved from beneath the enclosure during submergence of the structure.Valve 46 in conduit 45 controls flow of air therethrough. Conduit 47communicates with the lower interior space enveloped by enclosure 40 andflow of sea water therethrough is controlled by valve 48. The lower endsof radial tubular members 41 are sealed off by plates 39 so that suchmembers provide buoyancy for enclosure 40.

The enclosure of FIG.: possesses inherent stability when floating on asea surface because of the large cutwater plane, and the correspondingrighting moment thereby developed, through the presence of the largeradial tubular members 41. The enclosure of FIG. 5 can be submerged inthe same way as described with respect to the enclosure of FIG. 1.Furthermore, to aid in submerging enclosure 40 or to ballast it to morefirmly seat it on a sea floor, a conduit 49 and a conduit 50 areprovided in communication with each'of radial tubular members 41.Conduit49 can be used to supply a ballasting material, such as water orsand, to tubular member 41'and the" other conduit 50 can be employed tovent air therefrom during the ballasting operation.

Another embodiment of enclosure is illustrated by FIG. 6. The enclosure50 of'this embodiment is generally dome-shaped and approximates asemispherical shape. Enclosure 50 comprises a plurality of tubulartoroidal ribs or members 51 positioned parallel to one another in spacedapart horizontal arrangement. Adjacent tubular toroidalmembers 51 arejoined together by spherical shell segments 52 such as by means ofwelds. Each of the toroidal members 51 is advisably provided with twoconduits 53 and 54 in order that the 50 by conduit 55, sea water flowsin through conduit 57 and occupies the space.

The offshore liquid storage tank of FIG. 7 is adapted to rest underwateron a sea floor. The tank 60 has an enclosure 61 comprised of an innersemispherical metal shell 62 having a radius of 139 feet and aconcentrically positioned semispherical outer metal shell 63 with aradius of 148 feet. The space between the inner shell 62 and the outershell 63 is about 9 feet and such space is filled with a buoyantmaterial 70 and advisably with small hollow glass spheres which canstand a substantial compressive force. The bottom portion of enclosure61 has a ballasting ring wall 64 of concrete which is about 6 feet wideand 15 feet tall. Mounted on top of enclosure 61 is tubular column 65 ofabout 40 feet in diameter. On top of such column is platform 66. Theheight of the offshore structure of FIG. 7 from the top of the platformto the top of concrete ring wall 64 is about 290 feet.

With further reference to FIG. 7, conduit 67 communicates with the space68 enclosed or enveloped by enclosure 61. Conduit 67 can be used towithdraw air from space 68 during submergence of the offshore storagestructure and the flow of air therefrom can be controlled by valve 69therein. After the tank has been positioned on the sea floor, conduit 67can be used to feed interior ofeach tubular toroidal member 51'can beballasted. Thus, a solid or liquid ballasting material can be suppliedby conduit 53 and air vented therefrom by with air entrapped below theenclosure. The bottom of I enclosure is open to the sea so that, whenair is' vented therefrom by means of conduit with valve 56 open, theenclosure will slowly settle or submerge. When all air has been removedfrom the space enveloped by enclosure 50, and that space is fullyoccupied by sea water, the enclosure will have sufficient buoy- 'ancy tobe maintained at a floating level with only a small upper portion of theenclosure above sea level. Further submergence of the enclosure can beachieved with controlled stability as described with reference toFIG. 1. Once the enclosure has been positioned on the sea floor, conduit57 with valve 58 opened, can be employed to control flow of sea water toand from the space enveloped by the enclosure. To use the structure forthe storage of oil, oil can be pumped in through valve 56 and conduit55' and sea water displaced from beneath the enclosure through conduit57 with valve 58 open- When oil is removed from beneath enclosure oilinto space 68 for storage therein. Conduit 74 communicates with thelower portion of space 68 enveloped by enclosure 61 and, with valve 75open, permits sea water to flow in and out of space 68.When oil is fedto the space 68 through conduit 67,'sea water is displaced from space 68through conduit 74 and when oil is withdrawn from space 68 throughconduit 67, sea water enters through conduit 74.

The offshore oil storage structure of'FIG. 7 floats on water withminimum draft. The weight of the structure is borne on the air entrappedin space 68 and uncontrolled tilting and capsizing of the structure isavoided because of the substantial cut-water plane and righting momentexerted by the wall widthor thickness of enclosure 61. Submerg'ing ofthe structure is achieved by venting air from space 68 by conduit 67 andopening valves 73 and 76 so that buoyancy is transferred from thetrapped air in space 68 to the buoyant material 70 in space 61. Suitablemeans such as a screen through which water or air will pass but not thebuoyancy material 70 is positioned in conduits 72 and 77 where they joinouter shell 63. After all of the air has been removed from space 68,valve 69 is closed. After'the structure load is transferred to thebuoyant material 70,

the structure will still float due to the buoyancy provided by thebuoyant material in the space between inner and outer shells 62 and 63although at such point nearly all of enclosure 61 will be below thesurface of the sea.

Further submerging of the structure of FIG. 7 is achieved by openingvalve 73 and removing the screen in conduit 72 so that sea water canflow through conduit 77 into the space between shells 62 and 63 and thebuoyant material 70 can flow out through valve 73. The release ofbuoyant material is controlled so that the descent of the structure tothe sea floor is always under control. As column is pulled into thewater, it provides stability for the structure. When the structure isfinally deposited on the sea floor, all the buoyant material inenclosure 61 is released. The enclosure 61 can then be filled withballast in the form of stone, sand 7 or concrete supplied throughconduit 72 or such other conduits or openings in enclosure 61 as aremade for this purpose.

FIGS. 8 and 9 illustrate a further embodiment of an offshore structureprovided by this invention. The offshore structure of these figuresemploys an enclosure 80 which has an internal semispherical metal shell81 with a radius of 139 feet and an outer semispherical shell 82 with aradius of 144 feet. Thus, the inner shell 81 and the outer shell 82 arespaced feet apart. Enclosure 80 has a concrete ring wall 83 about 40feet high extending around the lower portion of the enclosure betweeninner shell 81 and outer shell 82. Ringlike airtight compartments 84,85, 86 and 87 are defined by spaced apart metal plate partitions 88, 89,90 and 91 positioned between, and extending to, inner shell 81 and outershell 82. Platform 92 having a 200 feet diameter is supported by aplurality of legs 93 on top of enclosure 80. Conduit 94 is employed forremoving air from space 955 encompassed within and below enclosure 80.Air.is vented therefrom by conduit 94 with-valve 95. open in submergingthe offshore oil storage structure.

The offshore oil storage structure of FIGS. 8 and 9 floats on the seawith about a 9 foot draft with the space 95 pressurized to about 4 psig.The compartments 84, 85, 86 and 87 are pressurized with air to aboutpsig 97 supported by brackets 98 mounted on the lower outer portion ofouter shell 82. The piles can be grouted in place to the pile-receivingsleeves for excellent bonding. It should be understood that the offshoreoil storage structure of FIGS. 8 and 9 is provided with such conduitsandvalve arrangements as are required to utilize it for its intendedpurpose.

The tank of this invention will normally have enclosures ballasted atthe bottom of the wall, although ballast is not needed in theory,because the ballast will help to anchor the enclosure in place on thesea floor and help to avoid a need for the use of anchoring pilesespecially when all or part of the buoyancy space in the enclosure willis filled with ballast, such as after it is on the sea floor. The use ofballast on the bottom of the enclosure also aids in placing the centerof gravity closer to the center of buoyancy, even though it advisably isstill above the center of buoyancy when the tank has maximum floatation,to aid in the submerging operation.

The foregoing detailed description has been given for clearness ofunderstanding only, and no unnecessary initially. Once the structure hasbeen floated to the installation area, the air in space 95 is ventedthrough conduit 94 until the partition 88 is about 45 feet below sealevel. At such point, compartment 84 will be subjected to a net externalpressure of about 10 psig. The pressure in compartment 84 is then raisedto about 20 psig following which more air is vented through conduit 94until the partition 88 is about 67 feet below sea level. At this point,compartment 84 is again at a net external pressure of 10 psig.Submergence is continued in this manner so that the net pressure on anyof comlimitations shoud be understood therefrom as modifications will beobvious to those skilled in the art.

What is claimed is:

1. An offshore storage tank to be placed on the floor of a body ofwather comprising:

a generally dome-shaped rigid continuous enclosure,

open at the bottom and surrounding a large volume, said enclosurecomprising an outer rigid semispherical shell and a smaller concentricinner semispherical shell, said shells being held in fixed positionrelative to each other by means of rigid connecting members joined toeach shell, and ballast means closing the bottom of the enclosurebetween the shell ends, thereby defining a hollow enclosed space betweenthe shells;

said hollow enclosed space constituting buoyancy means for providingbuoyancy balance or equilibrium to the tank when the tank is floating onwater with or without any amount of air in the large volume surroundedby the enclosure; and

conduit means communicating with the volume beneath the enclosure forremoving air therefrom,

whereby the enclosure can be lowered in water.

1. An offshore storage tank to be placed on the floor of a body ofwather comprising: a generally dome-shaped rigid continuous enclosure,open at the bottom and surrounding a large volume, said enclosurecomprising an outer rigid semispherical shell and a smaller concentricinner semispherical shell, said shells being held in fixed positionrelative to each other by means of rigid connecting members joined toeach shell, and ballast means closing the bottom of the enclosurebetween the shell ends, thereby defining a hollow enclosed space betweenthe shells; said hollow enclosed space constituting buoyancy means forproviding buoyancy balance or equilibrium to the tank when the tank isfloating on water with or without any amount of air in the large volumesurrounded by the enclosure; and conduit means communicating with thevolume beneath the enclosure for removing air therefrom, whereby theenclosure can be lowered in water.